1. Field of the Invention
This invention relates generally to a method and apparatus for monitoring receivers, and more particularly to a method and apparatus for determining the channel to which a receiver is tuned.
2. Description of the Prior Art
Various arrangements have been employed to determine the channel to which a television receiver is tuned. A disadvantage of many known monitoring arrangements is that an internal connection to the television receiver is required. For example, U.S. Pat. No. 3,973,206 issued to Haselwood, et al. on Aug. 3, 1976 discloses a method of determining channel tuning by monitoring the varactor diode tuning voltage and thus, required internal connections in the television receiver.
Another method for determining channel tuning disclosed in U.S. Pat. No. 4,425,578 issued to Haselwood et al. on Jan. 10, 1984 utilizes a signal injection source and a detector/receiver arranged to detect the combined signal from the injection signal source and a video carrier to which the video receiver is tuned. A significant drawback of signal injection monitoring arrangements is potential interference with the functional operation of the video receiver being monitored.
Other known arrangements have utilized a local oscillator frequency measurement of the monitored receiver to determine channel tuning. However, known local oscillator measurement arrangements are prone to errors caused by extraneous signals.